Sizing Guide

18 Sizing Guide www.hartandcooley.com
Type B Double-Wall Gas Vents
The configurations of the vent connector are not as
important as the connector rise and length require-
ments being satisfactorily met. All of the illustrated
methods in Figure 8 permit correct vent operation.
For economy, consider all alternatives
It is important in a combined vent system that the cost
of individual versus combined vents be considered,
especially if the system is short or many fittings are
needed.
Frequently, individual vents will prove more
economical than a combined system in instances of
this type (Figure 9).
Self-venting connectors sized from single-appliance
vent tables
When a vent connector as a part of a combined vent
system has a rise of 5 feet or more, it can be installed as
though it were an individual vent by using the appropri-
ate Single-Appliance Vent Tables. It is important when
sizing self-venting connectors that allowances be made
for lateral length and the number of turns.
When in doubt use one size larger vent
It is neither possible nor practical in some cases to
anticipate all installation or operational contingencies
in designing a vent system. A safe rule is, when in
doubt use one size larger connectors and common
vents than required by the Vent Tables.
Size of interconnecting tees
Interconnecting tees must be the same size as the
common vent, as shown in Figure 10.
Use of Manifolds
Use of Line V Capacities for Manifold Sizing
A manifold is merely a vent system that is a horizontal
extension of the lower end of a common vent. The
connection of a manifold to a common vent may be
made by either a 90° elbow or tee. A manifold should
be sized as a common vent, using the combined total
capacity and applicable total height of the vent system.
The V lines in the common vent table must be reduced
by 10% to determine the capacity of the manifold and
common vent. There is also the horizontal length
limitation of 1.5 feet per inch of common vent that
must be followed.
Horizontal Versus Sloped Manifolds
Some codes require pitched or sloped manifolds. The
requirements for sloped manifolds or connectors is a
necessity for vent systems having low insulating values
where condensation may occur. Adequate connector rise
is necessary for proper venting of all appliances; therefore,
lateral manifolds should NOT be excessively sloped. Too
much manifold slope may cause insufficient connector
rise at the appliance farthest from the common vent,
increasing the chance of draft-hood spillage.
Figure 8
Figure 10
Figure 9
LEAST
PREFERRED
MOST
PREFERRED
SHORT COMBINED VENTS
MAY REQUIRE EXCESSIVE
FITTINGS, EXTRA LARGE
COMMON VENT
INDIVIDUAL VENTS CAN
PROVIDE BETTER VENTING
AT LESS COST
3-FT. RISE
3-FT. RISE
3-FT. RISE
SMALL
TEE WRONG
MUST BE
SAME SIZE AS
COMMON
VENT
CORRECT
TEE
INCORRECT